Abstract

The invention of scanning tunnelling microscopy (STM) a quarter of a century ago has revolutionized the study of physical and chemical surface properties. In a standard STM experiment electrons tunnel from a sharp metal tip through vacuum into the specimen surface and thus make its local electronic density of states experimentally accessible. This information can be used for Angström-scale imaging and spectroscopy.

In the present talk I will focus on experiments with a low-temperature scanning tunnelling microscope (STM) in which the vacuum gap between tip and sample is eliminated. Correspondingly, simple vacuum tunnelling through the STM junction gives way to more complex regimes of electron transport. This provides opportunities for new and fascinating experiments. Two examples will be discussed in detail.

In a first group of experiments, the vacuum gap is eliminated by reducing the tip-surface distance up to the point when contact between tip and surface is established. If, e.g., the tip approaches a surface-adsorbed molecule, a well-defined chemical bond between the tip apex and the molecule may be formed. This technique allows precise measurements of electron transport through molecules [1].

In a second group of experiments, the vacuum gap is eliminated by a condensed gas that fills the space between tip and surface. In the presence of this gas (e.g. molecular hydrogen) the STM generates a totally new image contrast which visualizes the geometric structure of the specimen (as revealed by its total electron density) instead of its local electronic structure (as revealed by the local density of states in the vicinity of the Fermi level) which is commonly measured in STM [2,3].

[1] Nanotechnology 19, 065401 (2008) [2] New Journal of Physics 10, 053012 (2008) [3] cond-mat arXiv:0910.5825